The Etrex (that it is connected to the VHF at the moment) documentation says:

'White: Data Out' (and I have written a '+' on the page here, and this is connected to the yellow wire from the NAVMAN)
'Black: Ground' (and I have written a '-' on the page here, and this is connected to the green wire from the NAVMAN)

I can't remember how I worked out that about the Etrex connections, but it works

Now I want to replace the Etrex with the GPS out from the RPi Openplotter using the NMEA 0183 to USB converter.

After very short Googling, is T for 'Talker' (or simplistically 'Output') & R for 'Listener' (or simplistically 'Input')?
I found this which seems helpful, and since I have only one GPS output and one GPS input I can just connect as per (b) above?